The present invention relates to communications systems, and more particularly, to echo suppression in bi-directional communications links.
In many communications systems, for example landline and wireless telephone systems, voice signals are often transmitted between two system users via a bi-directional communications link. In such systems, speech of a near-end user is typically detected by a near-end microphone at one end of the communications link and then transmitted over the link to a far-end loudspeaker for reproduction and presentation to a far-end user. Conversely, speech of the far-end user is detected by a far-end microphone and then transmitted via the communications link to a near-end loudspeaker for reproduction and presentation to the near-end user. At either end of the communications link, loudspeaker output detected by a proximate microphone may be inadvertently transmitted back over the communications link, resulting in what may be unacceptably disruptive feedback, or echo, from a user perspective.
Therefore, in order to avoid transmission of such undesirable echo signals, the microphone acoustic input should be isolated from loudspeaker output as much as possible. With a conventional telephone handset, in which the handset microphone is situated close to the user's mouth while the handset speaker essentially covers the user's ear, the requisite isolation is easily achieved. However, as the physical size of portable telephones has decreased, and as hands-free speaker-phones have become more popular, manufacturers have moved toward designs in which the acoustic path from the loudspeaker to the microphone is not blocked by the user's head or body. As a result, the need for more sophisticated echo suppression techniques has become paramount in modern systems.
The need is particularly pronounced in the case of hands-free automobile telephones, where the closed vehicular environment can cause multiple reflections of a loudspeaker signal to be coupled back to a high-gain hands-free microphone. Movement of the user in the vehicle and changes in the relative directions and strengths of the echo signals, for example as windows are opened and closed or as the user moves his head while driving, further complicate the task of echo suppression in the automobile environment. Additionally, more recently developed digital telephones process speech signals through voice encoders which introduce significant signal delays and create non-linear signal distortions. Such prolonged delays tend to magnify the problem of signal echo from a user perspective, and the additional non-linear distortions make echo suppression by the network equipment more difficult.
In response to the above described challenges, telephone manufacturers have developed a wide variety of echo suppression mechanisms. An exemplary echo suppression system 100 is depicted in FIG. 1A. As shown, the exemplary system 100 includes a microphone 110, a loudspeaker 120 and an echo suppressor 130. An audio output 115 of the microphone 110 is coupled to an audio input of the echo suppressor 130, and an audio output 135 of the echo suppressor 130 serves as a near-end audio input to a telephone (not shown). Additionally, a far-end audio output 125 from the telephone is coupled to an audio input of the loudspeaker 120 and to a reference input of the echo suppressor 130.
In operation, the echo suppressor 130 processes the microphone signal 115 to provide the audio output signal 135 to a far-end telephone user. More specifically, the echo suppressor 130 attenuates the microphone signal 115, in dependence upon the far-end audio signal 125, so that acoustic echo from the loudspeaker 120 to the microphone 110 is not passed back to the far-end telephone user.
Typically, the echo suppressor 130 is either a non-linear, clipping type suppressor or a linear, scaling type suppressor. Clipping type suppressors generally attenuate the microphone output signal 115 by removing a portion of the signal falling within a particular range of values (i.e., within a particular clipping window). Scaling type suppressors, on the other hand, attenuate the microphone output signal 115 by multiplying the signal with an appropriate scale factor. Recently developed hybrid suppressors incorporate both clipping and scaling aspects, for example by scaling a portion of the microphone signal falling within a particular attenuation window. In any case, the level of attenuation (i.e., the clipping window and/or the scale factor) is generally adjusted, either directly or indirectly, in accordance with the amplitude of the far-end audio signal 125 so that the microphone output 115 is attenuated only to the extent the far-end user is speaking.
A conventional clipping type suppressor, known in the art as a center clipper, is described for example in U.S. Pat. No. 5,475,731, entitled "Echo-Canceling System and Method Using Echo Estimate to Modify Error Signal" and issued Dec. 12, 1995 to Rasmusson et al. An alternative clipping type suppressor, known as an AC-Center clipper, is described in copending U.S. patent application Ser. No. 08/775,797, entitled "An AC-Center Clipper for Noise and Echo Suppression in a Communications System" and filed Dec. 31, 1996. An exemplary scaling type suppressor is described in U.S. Pat. No. 5,283,784, entitled "Echo Canceller Processing Techniques and Processing" and issued Feb. 1, 1994 to Genter. An advanced hybrid suppressor, referred to herein as an AC-center attenuator, is described in copending U.S. patent application Ser. No. 09/005,149, entitled "Methods and Apparatus for Improved Echo Suppression in Communications Systems" and filed on even date herewith. Each of the above identified patents, as well as each of the above identified patent applications, is incorporated herein in its entirety by reference.
The echo suppressor 130 of FIG. 1A can also be combined with a linear echo canceler and/or a comfort noise generator to provide a more sophisticated echo suppression system. FIG. 1B depicts an exemplary system 101 including the microphone 110, the loudspeaker 120 and the echo suppressor 130 of FIG. 1A, and an acoustic echo canceler 140 and a comfort noise summing device 150. As shown, the microphone output 115 is coupled to an audio input of the acoustic echo canceler 140, and an audio output 145 of the acoustic echo canceler 140 is coupled to the audio input of the echo suppressor 130. The suppressor audio output 135 and a comfort noise signal are coupled to first and second inputs of the summing device 150, respectively, and an audio output 155 of the summing device 150 serves as the near-end audio input to the telephone (not shown). The far-end audio output 125 from the telephone is coupled to the audio input of the loudspeaker 120 and to reference inputs of the acoustic echo canceler 140 and the echo suppressor 130.
In operation, the acoustic echo canceler 140 dynamically models the acoustic path from the loudspeaker 120 to the microphone 110 and attempts to cancel, from the microphone output signal 115, any loudspeaker sound that is picked up by the microphone 110. Algorithms commonly used for modeling the acoustic echo path include the well known Least Mean Squares (LMS) algorithm and variants such as Normalized Least Mean Squares (NLMS). An exemplary Least Mean Squares based canceler is described in the above cited U.S. Pat. No. 5,475,731 to Rasmusson et al. Additionally, an advanced Normalized Least Mean Squares based canceler is described in copending U.S. patent application Ser. No. 08/852,729, entitled "An Improved Echo Canceler for use in Communications Systems" and filed May 7, 1997, which is incorporated herein in its entirety by reference.
In the system 101 of FIG. 1B, the echo suppressor 130 supplements the echo canceler 140 and provides additional echo attenuation as necessary. For example, the echo suppressor 130 can be dynamically adjusted based on a measurement of the instantaneous level of echo cancelation achieved by the acoustic echo canceler 140 to attenuate the residual echo to a predetermined goal level. Advanced methods for dynamically measuring the echo cancelation provided by an echo canceler are described, for example, in the above cited U.S. patent application Ser. No. 09/005,149 (entitled "Methods and Apparatus for Improved Echo Suppression in Communications Systems" and filed on even date herewith).
As in the system 100 of FIG. 1A, the echo suppressor 130 can be a clipping suppressor, a scaling suppressor or a hybrid suppressor. Additionally, the echo suppressor 130 can, when following the echo canceler 140, be a simple single-throw switch which selectively cuts out, or mutes, the audio output signal 135 at appropriate times (e.g., during periods in which a voice activity detector indicates that the microphone signal 115 contains no speech). In any case, the echo suppressor 130 attenuates the entire audio signal and, in addition to suppressing echo, distorts any background noise and/or near-end speech which may be present. In fact, the background noise can be suppressed to the point that the far-end user may erroneously believe that the call has been disconnected when the echo suppressor 130 is active.
Therefore, to improve the quality of communication for the far-end user, today's systems often add comfort noise to the telephone audio signal 135 when the echo suppressor 130 is active. In the system of FIG. 1B, comfort noise is added to the suppressor output signal 135 via the summing device 150. Alternately, the summing device 150 can be replaced with a switch so that either the suppressor output signal 135 or the comfort noise signal is selectively passed to the far-end user. Advanced methods for generating and utilizing comfort noise are described, for example, in U.S. patent application Ser. No. 09/005,145, entitled "Methods and Apparatus for Providing Comfort Noise in Communications Systems" and filed on even date herewith, which is incorporated herein in its entirety by reference.
While the above described systems provide sophisticated echo suppression, known methods for controlling such systems are inadequate in several respects. For example, known methods for controlling the echo suppressor 130 often result in the far-end user perceiving unacceptably distorted near-end speech. In other words, known methods of control sometimes activate the echo suppressor 130 when it should not be activated (e.g., during near-end single talk) and/or cause the echo suppressor 130 to attenuate the near-end signal more than is necessary or desirable (e.g., during periods of double talk). Additionally, certain known methods of controlling a residual echo suppressor (e.g., closing and opening a single-throw switch which follows a front-end echo canceler in dependence upon the presence or absence of near-end speech) can sometimes provide inadequate echo suppression. Consequently, there is a need for improved methods and apparatus for controlling echo suppressors.